An endoscope system that includes an endoscope unit, a processor unit, and a monitor is known as an endoscope system for observing a subject in a body cavity such as a person's esophagus or intestine. Illumination light emitted from the processor unit is guided in a light guide fiber bundle provided in the endoscope unit and emitted from a light distribution window provided at the distal end of the endoscope unit. The illumination light emitted from the light distribution window is reflected by a subject in the body cavity and is received as object light through an observation window provided at the distal end of the endoscope unit. The object light received through the observation window is converted into an electrical signal at a predetermined frame rate by an image sensor, and the electrical signal is output to the processor unit. The processor unit includes a signal processing circuit, and the received electrical signal is converted into an image signal by the signal processing circuit and transmitted to the monitor. The monitor displays a moving image based on the image signal received from the processor unit. Accordingly a user (operator) who is using the endoscope system is able to observe the subject by viewing the moving image displayed on the monitor. Also, if a freeze instruction for pausing (freezing) the moving image is input to the processor unit by the operator, a still image of the subject is displayed on the monitor. This allows the operator to inspect the subject. However, depending on the timing at which the operator inputs the freeze instruction, there have been cases where the still image displayed on the monitor is motion-blurred or defocused.
An endoscope system that is related to the aforementioned issue and suppresses color shift in a still image is disclosed in Japanese Patent No. 3887453 (referred to hereinafter as “Patent Document 1”). In the light source apparatus for an endoscope disclosed in Patent Document 1, a frame image in which color shift is lower than a predetermined value is selected from a moving image displayed at a predetermined frame rate, and is stored as a still image. Here, the number of still images that can be stored is limited, and the still images are updated in order of oldest image. In this endoscope system, when a freeze instruction is input, the still image of the frame that has the least amount of color shift is selected from among the stored still images, and is displayed. Accordingly, when a freeze instruction is input, a still image having little color shift is displayed on the monitor.